1. Field of the Invention
The present invention relates to a method for manufacturing a mask for focus monitoring, and also a method for manufacturing a semiconductor device.
2. Description of the Related Art
The importance of a focus monitoring technique in lithography has increased now that semiconductor devices have been so minute.
As the focus monitoring technique, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2001-100392 proposes the following method: a reference monitoring mark and another monitoring mark having a pattern shape corresponding to that of the reference monitoring mark are formed on a photomask, and after transferring (projecting) those monitoring marks to a wafer, the direction and amount of defocus are determined based on the difference in dimension between the monitoring marks transferred to the wafer. For example, the reference monitoring mark is set such that the phase difference between exposure light passing through an opening portion of the reference monitoring mark and exposure light passing through a surrounding portion surrounding the opening portion is 180°, and the other monitoring mark is set such that the phase difference between exposure light passing through an opening portion of the other monitoring mark and exposure light passing through a surrounding portion surrounding the opening portion is 90°. When those two kinds of monitoring marks are transferred to the wafer, they satisfy a relationship in which the difference in dimension between the marks or the ratio between their dimensions is proportional to a defocus amount. The above relationship can be predetermined and stored as data. Therefore, the direction and amount of defocus can be obtained by actually performing exposure to measure the dimension difference or dimension ratio.
As the method for setting the phase difference of the above other monitoring mark at 90°, there are provided a method for etching part of a quartz substrate (transparent substrate) which corresponds to the opening portion and a method for etching part of a halftone film (phase shift film) which corresponds to the surrounding portion. These methods both use an FIB (Focused Ion Beam) apparatus.
However, it should be noted that the number of directions in which the FIB apparatus can scan a beam is only two. To be more specific, it can scan the beam in horizontal and vertical directions only. It is therefore extremely difficult to process the quartz substrate or the halftone film by using the FIB apparatus in accordance with the pattern shape of the monitoring mark, since the monitoring mark is shaped to include part in which an edge of an opening portion extends in oblique directions, like a wedge or rhombus. In order to improve the accuracy of the processing, it is necessary to increase a processing amplification to shorten the pitch of scanning of the beam or reduce the diameter of the beam. However, if such a method is applied, it inevitably takes long time to perform the processing.
In such a manner, it is impossible for a conventional method to form a monitoring mark with a high precision for a short time period by using a charged beam such as an ion beam.